1. Field of the Invention
The present invention relates to electromagnetic wave absorbing structures and more particularly to multilayered or laminated electromagnetic wave absorbers with an extended frequency range, e.g., from under 30 Mhz up to 3 GHz and beyond to 18 GHz and above.
2. Description of the Prior Art
Electromagnetic wave absorbers made with a single layer of state-of-the-art ferrite achieve a vertical incidence/reflective performance of the order of 15 dB for frequencies from about 25 MHz to 600 MHz. For the EMC-frequency band, needing 30 MHz to 1 GHz (with provisions of up to 3 GHz), such a one-layer ferrite clearly lacks adequate performance. More appropriately, so-called superwide band absorbers make use of combinations of ferrite tiles and superposed flat layer materials on a metallic substrate in various known structural forms that achieve a relectivity R for vertical incidence better than 20 dB in a "superwide band" from 30 MHz to 3 GHz. However, the design of these structures has to take into account six variables for each layer, which can vary independently, i.e., Epsilon', Epsilon" or Sigma, Mu', Mu", d, and f, so that two permittivity (.epsilon.) values, two permeability (.mu.) values, layer thickness, and wave frequency are involved for just one of the plurality of layers. Therefore, R optimization becomes more difficult for each additional layer, even in the simple case of isotropic materials and an ideal conductive substrate.
Attempts at computer simulations of these structures have difficulties with identifying physical approaches to optimization and rarely offer insights on the electromagnetic phenomena involved, as well as never making a correlation with the feasibility of the material. Many layered broadband absorbers utilizing magnetic materials have been described in various publications since 1945 when broadband absorptive materials became of interest as radar absorptive materials (RAM). More recently, an article by Kim, Kim, and Hong in IEEE Transactions on Magnetics, Vol. 29, #3, July 1993, pps. 2134-2138, entitled, "A study on the Behavior of Laminated Electromagnetic Wave Absorbers", describes a double "ferrite against ferrite" layer absorber, where the center frequency of the absorber is changed between the frequencies of each ferrite. Further, U.S. Pat. No. 5,323,160, issued Jun. 21, 1994 to the foregoing authors, describes the same absorber wherein the "materials have different attenuation characteristics and are affixed to each other". Related magnetic spectra are shown by the same authors in the publication, J. Korean Institute Telemat. Electron. (South Korea), Vol. 28a, #8, pps. 9-14, August 1991, "Behavior of Laminates of Ferrites on Electromagnetic Wave Absorbers".
It has been observed that such qualities and implementations do not broaden and enhance the useful absorptive spectrum, where the em wave reflection is reduced by 20 dB, for instance. In fact two ferrites that have been considered have shown different magnetic permeability mu* spectra in the 30 MHz 1 GHz range but very similar mu* spectra in the 1 GHz range. With such structures no extension of this frequency is physically possible.
It is therefore a problem in the art to achieve a desirable range of useful absorptive spectra with current multilayered, electromagnetic wave absorbers.
It is accordingly an object of the present invention to provide an improved layered broadband electromagnetic wave absorber with a broadened and enhanced useful absorptive spectrum.
It is another object of the invention to provide an improved multilayered electromagnetic wave absorber with an extended frequency range from under 30 Mhz up to 3 GHz and beyond to 18 GHz and above.
It is a further object of the invention to provide an improved multilayered, electromagnetic wave absorber with a broadened and enhanced useful absorptive spectrum above 1 GHz with different mu* spectra for the layer materials and magnetic loss dispersion spectra.
It is also an object of the invention to provide an improved multilayered, electromagnetic wave absorber with a broadened and enhanced useful absorptive spectrum above 1 GHz and "decoupling" of the layers, i.e., reducing of the inherent coupling effect between the adjacent layers, by one or more of the following ways:
increasing the relative wave impedances by staggering the permittivity spectra of the layers; PA1 reducing the dielectric constants from the inside (against the ground reference) to the outside; PA1 separating the permeability dispersion loss spectra in the frequency domain; or PA1 adding intermediary layers of high relative wave impedance, including air layers.
It is an additional object of the invention to provide an improved multilayered, electromagnetic wave absorber with two ferrite layer implementations involving 30 MHz to 3 GHz absorbers using ferrites with different dispersion spectra and using one of the several dispersion techniques above.
It is an additional object of the invention to provide an improved multilayered, electromagnetic wave absorber with a number of two ferrite layer implementations of 30 MHz to 18 GHz absorbers using ferrites with well separated dispersion spectra and one of the several dispersion techniques above.
It is an additional object of the invention to provide an improved multilayered, electromagnetic wave absorber with a broadened and enhanced useful absorptive spectrum below 30 MHz where the coupling effects are small using the above techniques.
It is an additional object of the invention to provide an improved multilayered, electromagnetic wave absorber using a number of "smart materials", i.e., new ferri- and/or ferromagnetic materials as contemplated and synthesized in view of the desired or needed permeability and permittivity spectra, following the above rules of staggered magnetic dispersive loss spectra and/or the rules of decoupling of the successive layers of the absorbers.